Method for modifying the surface of a thermal barrier coating by plasma-heating

ABSTRACT

A method for providing a substantially-smooth protective coating on a metal-based substrate is disclosed. A thermal barrier coating is first applied over the substrate by plasma-spraying. The coating is then plasma-heated according to a time- and temperature schedule sufficient to re-melt its surface region, allowing the coating material to flow and smoothen. The surface region is then allowed to cool to a temperature below its melting point. After being cooled, the surface is much smoother than when originally applied, which allows the coating to be used in a higher-temperature environment. The coating is often zirconia-based, e.g., yttria-stabilized zirconia.

TECHNICAL FIELD

This invention relates generally to protective coatings applied tometals. More specifically, it is directed to modifying the surface ofsuch coatings, to beneficially alter some of their properties, such asheat transfer characteristics.

BACKGROUND OF THE INVENTION

Thermal barrier coatings (TBCs) are often used to improve the efficiencyand performance of metal parts which are exposed to high temperatures.Aircraft engines and land-based turbines are made from such parts. Thecombustion gas temperatures present in turbines are maintained as highas possible for operating efficiency. Turbine blades and other elementsof the engine are usually made of alloys which can resist the hightemperature environment, e.g., superalloys, which have an operatingtemperature limit of about 1000° C.-1150° C. Operation above thesetemperatures may cause the various turbine elements to fail and damagethe engine.

The thermal barrier coatings effectively increase the operatingtemperature of the turbine by maintaining or reducing the surfacetemperature of the alloys used to form the various engine components.Most thermal barrier coatings are ceramic-based, e.g., based on amaterial like zirconia (zirconium oxide), which is usually chemicallystabilized with another material such as yttria. For a turbine, thecoatings are applied to various surfaces, such as turbine blades andvanes, combustor liners, and combustor nozzles. Usually, the thermalbarrier coating ceramics are applied to an intervening bond layer whichhas been applied directly to the surface of the metal part.

The thermal barrier coatings are often applied to the part by a thermalspray technique, such as a plasma spray process. In this technique, anelectric arc is typically used to heat various gasses, such as air,oxygen, nitrogen, argon, helium, or hydrogen, to temperatures of about8000° C. or greater. (When the process is carried out in an airenvironment, it is often referred to as air plasma spray or "APS".) Thegasses are expelled from an annulus at high velocity, creating acharacteristic thermal plume. Powder material (e.g., the zirconia-basedcomposition) is fed into the plume, and the melted particles areaccelerated toward the substrate being coated. For some applications,plasma-spray techniques have numerous advantages over other coatingtechniques, such as electron beam physical vapor deposition (EB-PVD). Asan example, plasma spray systems are usually less costly than EB-PVD.Moreover, they are well suited for coating large parts, with maximumcontrol over the thickness and uniformity of the coatings.

Despite the advantages associated with plasma-sprayed thermal barriercoatings, the use of these processes can present some problems undervarious circumstances. For example, a plasma-sprayed coating often has arelatively rough surface, e.g., an "R_(a) " (arithmetic roughnessaverage) value greater than about 600 micro-inches. Much smoothersurfaces are required when the coating is to be applied to turbinecomponents like airfoils, so that the convective component of the heatflux delivered to the coating can be reduced. Moreover, the aerodynamicdrag losses can be also be reduced.

The thermal barrier coating surface can be smoothed by severaltechniques, such as grinding, tumbling, or heavy-sanding operations.However, these processes can be very time-consuming, adding considerablyto the overall cost of fabrication. Moreover, they can sometimesmechanically damage the thermal barrier coating. For example, asand-tumbling operation can sometimes result in the preferentialsmoothing/wearing of certain areas of the coating. The decreasedthickness in those areas can undesirably lower the thermal resistance ofthe thermal barrier coating. Grinding, on the other hand, can inducestresses in the coating, thereby reducing its service life.

Surface-smoothing processes for thermal barrier coatings have beenpracticed in the art. For example, H. L. Tsai et al describe the use ofa continuous wave laser to glaze the surface layer of a plasma-sprayedcoating based on yttria-stabilized zirconia (Materials Science andEngineering, A161 (1993), 145-155). The process is said to be capable ofproducing shiny surfaces of low roughness. However, a laser system canbe a considerable capital investment, adding to the cost and complexityof the overall thermal barrier coating process. Moreover, it maysometimes be quite difficult to adjust the wavelength of the laser tomelt the most appropriate surface-portion of the thermal barriercoating, i.e., a layer thick enough to form a smooth surface, but thinenough to preserve the overall integrity of the protective coating.

From this discussion, it should be apparent that new methods formodifying the surface of a thermal barrier coating would be welcome inthe art. The new processes should smooth the surface to a degreesuitable for aerodynamic applications, while maintaining all of thebeneficial characteristics of the coating. Moreover, the processesshould be fully compatible with the application of the thermal barriercoating over a substrate, and should not add excessive cost or time tothe overall production operation.

SUMMARY OF THE INVENTION

This invention is directed to improvements in the state-of-the artpresented above. In one aspect, the invention embraces a method forproviding a substantially-smooth protective coaling on a metal-basedsubstrate, comprising the steps of:

(a) applying a thermal barrier coating over the substrate byplasma-spraying;

(b) plasma-heating the applied coating according to a time- andtemperature schedule sufficient to re-melt the surface region of thecoating, allowing it to flow and smoothen; and then

(c) allowing the surface region to cool to a temperature below itsmelting point.

The thermal barrier coating is often zirconia-based, e.g.,yttria-stabilized zirconia, and may be applied over a conventional bondcoat layer. After the thermal barrier coating has been applied, theaction of the plasma torch, without coating particles being directedtherethrough, can be controlled to re-melt a surface region of thethermal barrier coating. The affected region usually has a thickness inthe range of about 1 micron to about 100 microns. After being cooled,the surface is much smoother than when originally applied, having asurface roughness (Ra) of less than about 250 micro-inches. The modifiedsurface beneficially reduces the amount of heat transfer into thethermal barrier coating, thereby allowing the coating to be used in ahigher-temperature environment. The modified surface also reducesaerodynamic losses. In the case of substrates which are part ofturbines, the aerodynamic enhancement results in higher turbineefficiency.

Other details regarding the various embodiments of this invention areprovided below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a photomicrograph of a cross-section of a thermal barriercoating applied to a metallic bond coat on top of a superalloysubstrate.

FIG. 2 is a photomicrograph of the coating of FIG. 1, after modificationaccording to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

A variety of materials can be used for the thermal barrier coating ofthis invention. In preferred embodiments, the coating is zirconia-based.Zirconia is a well-known compound for barrier coatings, and isdescribed, for example, in Kirk-Othmer's Encyclopedia of ChemicalTechnology, 3rd Edition, V. 24, pp. 882-883 (1984). In preferredembodiments, the zirconia is chemically stabilized by being blended witha material such as yttrium oxide (yttria), calcium oxide, magnesiumoxide, cerium oxide, scandium oxide, or mixtures of any of thosematerials. In one specific example, zirconia can be blended with about1% by weight to about 20% by weight yttria (based on their combinedweight), and preferably, from about 3%-10% yttria.

The substrate can be any metallic material or alloy which is typicallyprotected by a thermal barrier coating. Often, the substrate is aheat-resistant alloy, e.g., a superalloy. Such materials are describedin various references, such as U.S. Pat. Nos. 5,399,313 and 4,116,723,both incorporated herein by reference. High temperature alloys are alsogenerally described in Kirk-Othmer's Encyclopedia of ChemicalTechnology, 3rd Edition, Vol. 12, pp. 417-479 (1980), and Vol. 15, pp.787-800 (1981). Illustrative nickel-based alloys are designated by thetrade-names Inconel®, Nimonic®, Rene® (e.g., Rene® 80-, Rene® 95alloys), and Udimet®. As mentioned above, the type of substrate can varywidely, but it is often in the form of a turbine part, such as anairfoil component.

It is often desirable to apply a bond coat between the substrate and thethermal barrier coating to enhance adhesion to the substrate. The bondcoat is usually formed from a material like "MCrAIY", where "M"represents a metal like iron, nickel, or cobalt. It may be applied by avariety of conventional techniques, such as PVD; plasma spray or otherthermal spray deposition methods such as HVOF (high velocity oxy-fuel),detonation, or wire spray; CVD (chemical vapor deposition); orcombinations of plasma spray and CVD techniques. In some preferredembodiments, a plasma spray technique, such as that used for the thermalbarrier coating, is employed to deposit the bond coat layer. Usually,the bond coat has a thickness in the range of about 25 microns to about500 microns, and preferably, in the range of about 125 microns to about375 microns.

Various types of plasma-spray techniques may be utilized to apply thethermal barrier coating of the present invention. They are generallywell-known in the art, e.g., see the Kirk-Othmer Encyclopedia ofChemical Technology, 3rd Edition, V. 15, page 255, and references notedtherein. U.S. Pat. Nos. 5,332,598; 5,047,612 (Savkar and Lillquist); andU.S. Pat. No. 4,741,286 are instructive in regard to various aspects ofplasma spraying, and are incorporated herein by reference. In general,the typical plasma spray techniques involve the formation of ahigh-temperature plasma, which produces a thermal plume. The coatingmaterial, e.g., zirconia powder, is fed into the plume, and thehigh-velocity plume is directed toward the substrate. In preferredembodiments, an air plasma spray technique is used.

Those of ordinary skill in the plasma spray coating art are familiarwith various details which are relevant to applying the coating.Examples of the various relevant steps and process parameters include:Cleaning of the surface prior to deposition; grit blasting to removeoxides and roughen the surface; substrate temperature; plasma sprayparameters such as spray distances (gun-to-substrate), selection of thenumber of spray-passes, powder feed rate, particle velocity, torchpower, plasma gas selection, oxidation control to adjust oxidestoichiometry, angle-of-deposition, post-treatment of the appliedcoating; and the like.

Torch power may vary in the range of about 10 kilowatts to about 200kilowatts, and in preferred embodiments, ranges from about 40 kilowattsto about 60 kilowatts.

The velocity of the zirconia particles flowing into the plasma plume (orplasma "jet") is another parameter which is usually controlled veryclosely. To briefly review (and as described in several of thereferences, e.g., U.S. Pat. No. 5,047,612), the typical plasma spraysystem includes a plasma gun anode which has a nozzle pointed in thedirection of the deposit-surface of the substrate being coated. Theplasma gun is often controlled automatically, e.g., by a roboticmechanism, which is capable of moving the gun in various patterns acrossthe substrate surface.

The plasma plume extends in an axial direction between the exit of theplasma gun anode and the substrate surface. Some sort of powderinjection means is disposed at a predetermined, desired axial locationbetween the anode and the substrate surface. In some preferredembodiments, the powder injection means is spaced apart in a radialsense from the plasma plume region, and an injector tube for the powdermaterial is situated in a position so that it can direct the powder intothe plasma plume at a desired angle. The powder particles, entrained ina carrier gas, are propelled through the injector and into the plasmaplume. The particles are then heated in the plasma and propelled towardthe substrate. The particles melt, impact on the substrate, and quicklycool to form the thermal barrier coating.

The thickness of the thermal barrier coating will depend on the end useof the part being coated. Usually, the thickness is in the range ofabout 125 microns to about 2500 microns. In preferred embodiments forend uses such as airfoil components, the thickness is often in the rangeof about 250 microns to about 750 microns.

After the thermal barrier coating has been applied, it is plasma-heatedaccording to a time- and temperature schedule sufficient to re-melt thesurface region of the coating. A variety of plasma systems can beemployed for this purpose. In preferred embodiments, the plasma systemis that which was used to apply the thermal barrier coating. Forexample, the powder feed to the system could be shut off, while theplasma plume continues to be directed toward the substrate.

As used herein the term "surface region" of the coating is usually acoating thickness in the range of about 1 micron to about 100 microns.The thermal-insulating capability of the thermal barrier coatingmaterial usually prevents melting from occurring at greater depths. Ingeneral, it is preferred that melting occur through as little a regionas is necessary to achieve the effect of smoothing the surface, e.g.,melting a surface region having a thickness less than about 50 microns,and preferably, less than about 30 microns. (The desired melting depthwill depend in part on the degree of surface roughness which wasinitially present.).

The temperature required to re-melt the surface region of the thermalbarrier coating depends in part on the composition of the coating, i.e.,the overall melting point of the composition. In the case of ayttria-stabilized zirconia-based coating, the required temperature isusually at least about 2750° C. In general, the temperature at thesurface should be the minimum temperature (above the melting point)which will allow flow of the molten material. Those skilled in the artcan readily determine the temperature needed for various types ofthermal barrier coatings, based on available melting point data.

It is known in the art that plasma temperatures themselves, i.e., withinthe thermal plume, are very high, e.g., about 10,000° C. For a typicalplasma system, the distance from the plasma torch to the applied coatingduring the re-melting step will depend on the type of plasma gun, thegun speed over the part, the power of the gun and the plasma conditionsused. These distances could vary from 0.5 cm to about 17 cm and willdepend on the gun and the choice of the conditions mentioned above.Adjustments in that range can be made, depending on many of the factorsset forth above, e.g., torch power, torch speed, thermal barrier coatingcomposition, and the like. Moreover, gas flow into the plasma can beadjusted to reduce the occurrence of molten surface material"splashing", which could otherwise lead to a wavy surface after cooling.

Often, the substrate surface or "target" is positioned verticallyrelative to the ground, and the torch is moved across the surface fromleft-to-right or right-to-left. The torch is indexed downwardly orupwardly, depending on what part of the surface is heated first. Torchspeed will depend on many of the factors mentioned above, but usually isin the range of about 250 cm per minute to about 7600 cm per minute. Ahigh torch speed (within this range) could be used when the distancefrom the torch to the coating is relatively close, while a lower torchspeed could be used when the distance is greater.

After plasma-heating is stopped, the surface region of the substratewill quickly cool to a temperature below its melting point. Usually, thetime required for cooling will be less than about 1 second.

As shown below in the examples, the treated surface becomes very smooth.Usually, the surface roughness will be less than about 250 micro-inchesRa after being plasma-heated. Often, the surface roughness is less thanabout 150 micro-inches Ra. The modified surface beneficially reduces theamount of heat transfer into the thermal barrier coating.

It should be apparent from the above discussion that another aspect ofthis invention is directed to a heat-resistant metal article having aplasma-smoothened surface formed from a thermal barrier coating. Thesurface has a roughness (Ra) of less than about 250 micro-inches, andpreferably, less than about 150 micro-inches. The substrate may beformed of a superalloy material, and is sometimes covered with a bondcoat which is positioned below the thermal barrier coating.

EXAMPLE

The example which follows illustrates some embodiments of thisinvention, and should not be construed to be any sort of limitation onits scope.

The test sample was a coupon made from a nickel-based superalloy, Rene®N-5, having a dimension of 1 inch (2.5 cm)×2 inches (5.0 cm), with athickness of 0.125 inch (0.32 cm). Prior to deposition of the bond coat,the coupon was grit-blasted (60 grit) and then ultrasonically cleanedwith an alcohol and acetone. A bond coat of the NiCrAlY-type was firstapplied to the coupon-substrate, using an air plasma system. Thethickness of the bond coat was about 250 microns.

A thermal barrier coating (zirconia, with 8 wt. % by weight yttria)having an average thickness of about 500 microns, was then airplasma-sprayed onto the bond coat. A commercially-available Metco spraygun system, robotically controlled, was used to deposit the coatings.The substrate was positioned vertically relative to the ground, and theplasma torch was moved horizontally across the surface, and then indexedvertically, to cover the entire surface area. The deposited moltendroplets solidify very shortly after contact with the depositionsurface.

Surface roughness was measured by way of stylus profilometry. Themeasurements were taken (three times each) in the same direction as themotion of the plasma gun, and also in a direction perpendicular to thatdirection. The resulting measurements were then averaged.

FIG. 1 is a photomicrograph showing a cross-sectional depiction of thesample, at a magnification of 200×. The average roughness of the thermalbarrier coating (Ra) was 700 micro-inches.

With the powder feed turned off, the plasma torch was again passed overthe surface according to the same pattern. The torch speed was about7100 cm per minute, and the torch-to-substrate distance was about 1.9cm, which was sufficient to bring a portion of the thermal barriercoating material to its liquidus temperature. Re-melting within thethermal barrier coating occurred to a depth of about 25 microns. Soonafter the plasma torch has moved away from the melted surface, thetemperature of the surface drops below the melting temperature and thesurface resolidifies.

FIG. 2 is a photomicrograph showing a cross-sectional depiction of thesample at this stage. The smoothness of the sample surface is clear fromthe figure, and the measured, average roughness was less than about 120micro-inches Ra. It should be noted that the profilometry values weresometimes influenced by the presence of vertical cracks in the coatingsurface. These cracks are part of the desired microstructure of thecoating, and the profilometer needle registered them during traversal ofthe surface. If the presence of the vertical cracks is discounted, thesurface roughness measurements after treatment according to thisinvention would have been even lower.

While preferred embodiments have been set forth for the purpose ofillustration, the foregoing description should not be deemed to be alimitation on the scope of the invention. Accordingly, variousmodifications, adaptations, and alternatives may occur to one skilled inthe art without departing from the spirit and scope of the claimedinventive concept.

All of the patents, articles, and texts mentioned above are incorporatedherein by reference.

What is claimed:
 1. A method for providing a substantially-smoothprotective coating on a metal-based substrate, comprising the stepsof:(a) applying a thermal barrier coating over the substrate byplasma-spraying; (b) plasma-heating the applied coating according to atime- and temperature schedule sufficient to re-melt the surface regionof the coating, allowing it to flow and smoothen, said surface regionhaving a thickness in the range of about 1 micron to about 100 microns;and then (c) allowing the surface region to cool to a temperature belowits melting point.
 2. The method of claim 1, wherein the thermal barriercoating is zirconia-based.
 3. The method of claim 2, wherein the thermalbarrier coating comprises yttria-stabilized zirconia.
 4. The method ofclaim 2, wherein the plasma-heating temperature of step (b) is at leastabout 2750° C.
 5. The method of claim 1, wherein an air-plasma techniqueis used in steps (a) and (b).
 6. The method of claim 5, wherein gassesare expelled from a torch to form the plasma, and wherein the distancefrom the torch to the applied coating in step (b) is maintained in therange of about 0.5 cm to about 17 cm.
 7. The method of claim 6, whereinthe torch is moved over the applied coating in step (b) at an averagetraverse-torch speed of about 1800 cm per minute to about 7600 cm perminute.
 8. The method of claim 7, wherein the torch movement iscontrolled by a robotic mechanism.
 9. The method of claim 1, wherein thesurface region has a thickness in the range of about 1 micron to about50 microns.
 10. The method of claim 1, wherein the applied coating has asurface roughness (Ra) of less than about 250 micro-inches after step(c).
 11. The method of claim 10, wherein the applied coating has asurface roughness of less than about 150 micro-inches after step (c).12. The method of claim 1, wherein a metallic bond layer is applied onthe substrate prior to the application of the thermal barrier coating.13. The method of claim 12, wherein the bond layer is applied by athermal spray technique.
 14. The method of claim 13, wherein the thermalspray technique is an air plasma spray process.
 15. The method of claim1, wherein the substrate is a nickel-based superalloy.
 16. A process forpreparing a metal article which is resistant to high temperature,comprising the steps of:(i) applying a thermal barrier coating over thesubstrate with an air-plasma spray device which propels barrier coatingparticles onto the substrate; (ii) plasma-heating the thermal barriercoating with the air-plasma spray device without propelling barriercoating particles, according to a time- and temperature schedulesufficient to re-melt the surface region of the coating, allowing it toflow and smoothen, said surface region having a thickness in the rangeof about 1 micron to about 100 microns and then (iii) allowing thesurface region to cool to a temperature below its melting point.
 17. Theprocess of claim 16, wherein a metallic bond layer is applied on thesubstrate prior to the application of the thermal barrier coating. 18.The process of claim 16, wherein the thermal barrier coating comprisesyttria-stabilized zirconia.